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Without knowing what the contents of a book is going to be, one cannot make that book.
In other words, in order to make a book, you have to know which writings are going to go into that book, or you cannot make that book.
In order to make the "New Testament", they had to know which of the multitudes of the then-existing "writings", "gospels", "letters", etc., were going to be part of the "contents" of the New Testament, or they could not have made the New Testament. Those contents to be selected were not specified anywhere, in any scriptures. Those specific contents were all selected (out of many more that obviously existed) by men, under the guidance of the Holy Spirit (in "Church Councils").
If you don't understand how that was done, I urge you to read the following book:
"Where We Got the Bible... Our Debt to the Catholic Church" by Henry G. Graham
(And if you somehow think the list of writings which belong in the New Testament is contained in the Bible somewhere, tell me the Bible text(s) that list of writings is specified in.)
Do you think the Holy Spirit included a list somewhere in Scriptures which detailed which writings belong in the New Testament, or did God choose to use another means (external to the Scriptures) for having men select exactly which written documents belonged in the New Testament, and which written documents did not belong in the New Testament?
Don't you believe that the "New Testament" and its teachings are "critical for salvation and maturity in Christ"?